Technical Field
Embodiments described in the present disclosure are directed generally to catalytic heaters having replaceable catalytic cartridges for heating applications.
Description of the Related Art
A number of fluids that are normally found in gaseous form are commonly stored and transported under pressure as liquids, including, for example, methane, butane, propane, butadiene, propylene, and anhydrous ammonia. Additionally, fuel gasses comprising one or more constituent gasses are also stored and transported under pressure as liquids, including, e.g., liquefied petroleum gas (LPG), liquefied natural gas (LNG), and synthetic natural gas (SNG). Of these, LPG is perhaps the most commonly used. Accordingly, the discussion that follows, and the embodiments described, refer specifically to LPG. Nevertheless, it will be understood that the principles disclosed with reference to embodiments for use with LPG tanks can be similarly applied to tanks in which other liquefied gases are stored or transported, and are within the scope of the invention.
LPG is widely used for heating, cooking, agricultural applications, and air conditioning, especially in locations that do not have natural gas hookups available. In some remote locations, LPG is even used to power generators for electricity. LPG is typically held in pressurized tanks that are located outdoors and above ground. Under one atmosphere of pressure, the saturation temperature of LPG, i.e., the temperature at which it boils, is around −40° C. As pressure increases, so too does the saturation temperature. LPG is held in a liquid state by gas pressure inside the tank. As gas vapor is drawn from the tank for use, the pressure in the tank drops, allowing more of the liquefied gas to boil to vapor, which increases or maintains pressure in the tank.
As the gas boils, the phase change from liquid to gas draws thermal energy from the remaining liquid, which tends to reduce the temperature of the LPG in the tank. If LPG temperature drops, the boiling slows or stops, as the LPG temperature approaches the saturation temperature. Thus, boiling LPG tends to increase pressure and saturation temperature, while at the same time tending to decrease the actual temperature of the LPG in the tank, until an equilibrium temperature is reached, at which point the saturation temperature is equal to the current temperature of the LPG. Provided the energy expended to vaporize the gas does not exceed the thermal energy absorbed by the tank externally, from, for example, sunlight and the surrounding air, the LPG will continue to boil as vapor is drawn off, until the tank is empty. On the other hand, if more energy is expended to vaporize the gas than is replaced by external sources, the temperature in the tank will drop toward the equilibrium temperature, resulting in less energetic boiling, and a drop in tank pressure. If tank pressure drops too low, it can interfere with the operation of appliances and equipment that draw gas for use, such as furnaces, ovens, ranges, etc.
For purposes of the following disclosure, the maximum continuous rate at which gas can flow from a supply tank using only ambient energy to vaporize the LPG, without causing the tank pressure to drop below an acceptable level, will be referred to as the maximum unassisted flow rate. It will be recognized that this rate will vary according to the ambient temperature near the tank.
Low tank pressure is a particular concern in regions where ambient temperature can drop to very low levels, such as during the winter at high latitudes, or at very high altitudes. For example, when ambient temperature drops very low, the heat energy available to warm an LPG storage tank is reduced, while at the same time, the cold temperature prompts an increased draw of gas to fuel furnaces to warm homes and other buildings. As gas pressure drops below the regulated pressure of the gas line, flames in furnaces, water heaters, and other gas consuming appliances reduce in size, producing less heat and prompting users to open gas valves further, which only accelerates the pressure drop. Eventually, tank temperature can drop below the boiling point of unpressurized gas, at which point, no gas will flow. It can be seen that, as ambient temperature drops, the potential for unacceptable loss of pressure increases, as does the potential demand for gas, such as for heating.
Generally, disadvantages of many of the systems available are often related to the difficulty of providing heat in the close vicinity of an LPG tank without creating a condition that would be dangerous in the event of a tank leak or tank over-pressure. The complexity of systems in which a heat source is remotely located not only increases the cost, but also the likelihood of malfunction. Additionally, vaporizers and heaters that employ electric heating elements, or that are electrically controlled, are impractical for use in applications where electrical power is not available. In such cases, an electric generator is required to provide the electricity, resulting in costly efficiency losses.
One problem associated with electric tank heaters, in particular, is that the heating element is in direct contact with the tank wall. Temperature differentials between the element and the tank can promote water condensation, which can be trapped between the heating element and the surface of the tank, resulting in deterioration of the paint and subsequent corrosion of the steel tank wall. Most jurisdictions have stringent regulations regarding the use of combustion sources near LPG tanks and gas transmission lines. These regulations dictate explosion-proof requirements for electrical connections, minimum distances to open flames, etc. The restrictions vary according to the size of a tank and proximity to public areas.
One problem associated with other tank heaters, in particular, is that servicing the heater and replacing integral components can be burdensome and costly in situations where the entire heater or other component must be sent to an off-site location from the storage tank for servicing. As such, the heater will be out of commission during such servicing, which negatively affects delivery of the fuel in the tank to a load.